Control of the vertical position of the arc in a horizontally-operated HID lamp is desirable in a number of lighting applications, particularly automotive HID headlamps. The reasons are several-fold, and depend on the application. In horizontal HID lamps, the arc is bowed upwards by buoyancy induced gas convection in the arc tube. Straightening the arc can improve lamp life and color by both eliminating overheating of the top of the arc tube and increasing the temperature of the fill chemicals which reside at the bottom of the arc tube. In addition, a straightened arc provides a light source which is more compatible with optical collection and projection systems. It may also be desirable to move the light beam which is projected from the optical system by deflecting the arc in a controlled manner.
Several techniques have already been used to straighten arcs of horizontally operated lamps. These include the use of narrow-bore arc tubes to constrain the arc; excitation of acoustic resonances in the gas in the arc tube; rotating the gas in the arc tube about the arc axis either by flowing the gas, or rotating the entire arc tube; and applying a downward force on the gas by applying a horizontally oriented magnetic field which is perpendicular to the arc axis.
The force on the arc by a magnetic field is the vector cross product of the arc current and the magnetic field. If the arc axis and the magnetic field are perpendicular, and both lie in the horizontal plane, the force is vertical. The direction and magnitude of the force depends on the product of the magnitude and direction of the magnetic field and the arc current. To maintain a downward force on an alternating current arc, therefore, the magnetic field must be an alternating current synchronized with the arc current. This approach has been demonstrated and has been proposed as a product for automotive headlamps. The problem with this approach is that the equipment required to produce an alternating current magnetic field of sufficient magnitude is large, cumbersome, energy-consuming, and heat producing. Positioning the magnetic pole-pieces without negatively-impacting the design of the optical fixture is a challenge. Early attempts to achieve magnetic deflection with alternating current failed. The electromagnets were too slow. It was believed, that it was not possible to create fast and alternating magnetic fields to follow the signal of the alternating current ballast. Recently this has been overcome as shown in U.S. patent application Ser. No. 09/099,379.
By contrast, a direct current arc is easily straightened or deflected by a constant magnetic field of a modest level which is easily achieved even at a significant distance from a permanent magnet. However, direct current is not desirable in HID lamps because the current flow in one direction causes migration of the chemicals toward the cathode and results in separation of the various color components of the arc. Additionally, the anode and cathode electrodes must be designed differently, and the entire arc tube constructed asymmetrically to account for the unequal heat loads, and sealing difficulties.
To be practical, the whole magnetically deflected arc lamp system must be simple and reliable. The lamp, the ballast and the magnetic system must each be inexpensive to manufacture, efficient to operate and reliable. A simple alternating current lamp would be useful, but this has required the synchronized magnets described. A simple permanent magnet or electromagnet would be useful, but this has required the less desirable direct current lamps. The combination has not been possible. The present Applicant has now found a solution.